Dalton Transactions PAPER
Cite this: Dalton Trans., 2014, 43, 17624
Synthesis and photoinduced electron transfer in platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes† Sai-Ho Lee,a Chris Tsz-Leung Chan,b Keith Man-Chung Wong,a Wai Han Lam,a Wai-Ming Kwokb and Vivian Wing-Wah Yam*a Platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, were synthesized. Their photophysical properties were studied by electronic absorption and emission spectroscopy and the origin of the transitions was supported by computational studies. The electrochemical properties were also studied and the free energies for charge-separation and charge-recombination processes were evaluated. The photoinduced electron transfer reactions in the triads were investigated by femtosecond and nanosecond transient absorption spectroscopy. In
Received 12th May 2014, Accepted 8th August 2014 DOI: 10.1039/c4dt01397b www.rsc.org/dalton
dichloromethane, both triads undergo ultrafast charge separation from the 3MLCT/LLCT excited state within 300 fs to yield their respective triplet charge-separated (CS) states, namely (Cbz)2•+–Pt(bpy)–C60•− and (tBuCbz)2•+–Pt(bpy)–C60•−, and the CS states would undergo charge recombination to give the 3C60* state, which subsequently decays to the ground state in 22–28 μs.
Introduction The study of photoinduced electron transfer reactions in donor–acceptor systems has been an active area of research owing to their potential applications in artificial photosynthesis and photovoltaic devices.1 Donor–acceptor systems, particularly those utilizing fullerene as an electron acceptor, have attracted considerable attention in recent years.2 Fullerene is known to be an excellent electron acceptor due to its symmetrical shape, large size, and π-electron delocalization over the three-dimensional surface.2b,3 With small reorganization energy, efficient charge separation and slow charge recombination were accomplished by utilizing C60 as an electron acceptor. On the other hand, carbazole (Cbz), an aromatic amine, and its derivatives are known to be good electron donors4 and have been employed to develop a number of Cbz–C60 donor– acceptor systems with their photoinduced intramolecular electron transfer reactions well investigated.5 It has been demonstrated that the lifetime of the triplet charge-separated (CS) state in a Cbz–C60 dyad generated via 3C60* is significantly a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: The selected singlet excited states (Sn) computed by TDDFT/CPCM calculation, selected molecular orbitals and Cartesian coordinates of the optimized structures. See DOI: 10.1039/c4dt01397b
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longer than that of the singlet CS state generated via 1C60*,5b as charge recombination (CR) from the triplet CS state to the singlet ground state is a spin-forbidden process. One effective approach to develop Cbz–C60 systems that are capable of generating the triplet CS state would be to utilize transition metalbased chromophores for the development of donor–chromophore–acceptor triads. Transition metal-based chromophores which have low lying metal-to-ligand charge-transfer (MLCT) excited states, such as Ru(II),6 Re(I),7 Os(II),8 Ir(III),1b,6a,b,8g,9 Cu(I)10 and Pt(II),11 have been extensively studied. Owing to the ultrafast intersystem crossing (ISC) through the strong spin– orbit coupling imparted by the heavy metal centres, charge transfer processes generally occur at the 3MLCT excited state, and hence the spin-multiplicity of the CS state can be controlled. Unlike Ru(II),6 Re(I),7 Os(II),8 Ir(III),1b,6a,b,8g,9 and Cu(I),10 square-planar d8 Pt(II) based donor–acceptor systems are relatively less extensively studied. It was only until quite recently that Pt(II) based donor–acceptor systems have been explored for their photoinduced electron transfer properties,11 mainly encouraged by the development of the Pt(II) polypyridine alkynyl chromophores that display enhanced 3MLCT excited state behavior through the enlargement of the d–d orbital splitting by the incorporation of an alkynyl ligand that leads to a higher-lying d–d ligand-field state.12 In the present study, we develop a series of donor–chromophore–acceptor systems based on the bipyridine Pt(II) bisalkynyl chromophore. Herein, we report the synthesis, electrochemical and photophysical
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Electronic absorption and emission spectroscopy
Chart 1
studies of a series of donor–chromophore–acceptor triads (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 as well as their relevant model compounds, (Cbz)2–Pt(bpy), (tBuCbz)2–Pt(bpy), CbzC6H4–CuC–Ph and tBuCbzC6H4–CuC–Ph (Chart 1), using a variety of experimental techniques including electronic absorption and emission spectroscopy, cyclic voltammetry, femtosecond and nanosecond time-resolved transient absorption spectroscopy, and the photochemical processes are identified and the rate constants are determined. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to provide further insights into the origin of the electronic absorption and emission properties.
Results and discussion Synthesis The synthesis of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 is shown in Scheme 1. N-(4-Ethynylphenyl)carbazole and N(4-ethynylphenyl)-3,6-di-tert-butyl-carbazole were first prepared by the Ullmann-type reaction between carbazoles and 1-bromo-4-iodobenzene, followed by the palladium-catalyzed Sonagashira coupling with (trimethylsilyl)acetylene (TMSA), and the subsequent removal of the TMS group with potassium carbonate in tetrahydrofuran–methanol solution. On the other hand, according to literature procedures,13 the fullereneappended bipyridine derivative was obtained by the oxidation of 4,4′-dimethyl-2,2′-bipyridine with selenium oxide yielding 4-methyl-2,2′-bipyridine-4′-carboxaldehyde, followed by the incorporation of fullerene with the Prato reaction.14 The fullerene-appended bipyridine derivative was reacted with [PtCl2(DMSO)2] to afford the dichloroplatinum(II) precursor, which was then reacted with alkynes using copper-catalyzed coupling reaction to yield (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– Pt(bpy)–C60, with their identities confirmed by 1H NMR spectroscopy, FAB mass spectrometry and elemental analysis.
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The electronic absorption spectra of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) exhibit an intense absorption band at ca. 280–360 nm and a moderately intense band at ca. 360–450 nm with a λmax at ca. 400 nm in dichloromethane (Fig. 1). The high-energy intense absorptions are the π–π* transitions of the carbazole, bipyridine and alkynyl moieties, whereas the less intense absorptions in the visible region are attributed to an admixture of platinum-to-bipyridine metal-to-ligand charge transfer (MLCT) and alkynyl-to-bipyridine ligand-to-ligand charge transfer (LLCT) transitions.12b,i,j With the C60 moiety appended in (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, an absorption tail extending to 700 nm, which can also be observed in the electronic absorption spectrum of N-methylfulleropyrrolidine,3b is tentatively assigned to the π–π* transitions of C60. TDDFT calculations were performed on selected complexes (Cbz)2–Pt(bpy) and (Cbz)2–Pt(bpy)–C60 to provide further insight into the nature of the electronic transitions for the complexes. To reduce the computational cost, the dodecyl group of N-dodecylpyrrolidine on C60 for (Cbz)2–Pt(bpy)–C60 was replaced by a methyl group and was labeled as (Cbz)2–Pt(bpy)–C60′ (see Computational details). Selected singlet–singlet transitions for the complexes are listed in Table S1† and the molecular orbitals involved in the transitions are shown in Fig. 2 and S1.† The first two transitions computed at 484 and 451 nm for (Cbz)2–Pt(bpy) are predominantly the LLCT [π(CuC–Ph–Cbz)→π*(bpy)] transitions, slightly mixed with the MLCT[dπ(Pt)→π*(bpy)] character (Table S1†). For (Cbz)2–Pt(bpy)–C60′, the several transitions computed to be less intense in the lower-energy region are the IL π–π* transitions of the C60. The LLCT [π(CuC–Ph–Cbz)→π*(C60)] excited state is the S2 state which is close in energy to the lowest-energy IL(C60) state. When compared with (Cbz)2–Pt(bpy), the LLCT [π(CuC–Ph–Cbz)→π*(bpy)]/MLCT[dπ(Pt)→π*(bpy)] transitions for (Cbz)2–Pt(bpy)–C60′ are found to be red-shifted. On photoexcitation, (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) display observable emission in dichloromethane (Fig. 3), while fullerene-appended complexes (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– Pt(bpy)–C60 are non-emissive under identical conditions. The emission is red-shifted after the introduction of the di-tertbutyl groups at the 3,6-positions of carbazole in (tBuCbz)2–Pt(bpy). The emission of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) was found to be strongly quenched with their luminescence quantum yields determined to be 0.03 and 0.01, respectively, much lower than 0.50 for a platinum(II) diimine bis(alkynyl) chromophore.12a The weak emissive nature of the carbazolecontaining complexes may possibly be due to the occurrence of a quenching process in the excited state. A possible interpretation of this is that a competitive process of photoinduced electron transfer between the carbazole moiety and the Pt(II) bipyridine chromophore that leads to photochemical charge separation has taken place. Similarly, the nonemissive nature of the fullerene-containing complexes could again be ascribed to photoinduced electron transfer processes.
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Scheme 1 Synthesis of donor–chromophore–acceptor triads, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60. Reagents and conditions: (i) 4-bromoiodobenzene, K2CO3, 18-crown-6, Cu, DMF, reflux; (ii) PdCl2(PPh3)2, CuI, PPh3, TMSA, Et3N, reflux; (iii) K2CO3, MeOH–THF, r.t.; (iv) SeO2, dioxane, reflux; (v) N-dodecyl glycine, C60, PhMe, reflux; (vi) PtCl2(DMSO)2, CH2Cl2, r.t.; (vii) CuI, Et3N, CH2Cl2, r.t.
Electrochemistry In order to evaluate the free energy changes associated with the CS and CR processes in (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– Pt(bpy)–C60, cyclic voltammetry was carried out in 0.1 M n-Bu4NPF6 solution in deaerated dichloromethane. The first and second reduction couples in the triads are assigned to the reduction of the C60 moiety, with the observed reduction potentials E°(C60/C60•−) and E°(C60•−/C602−), at −0.59 V and −0.98 V (vs. SCE). Furthermore, the third reduction couple, which is assigned as the bipyridine ligand-centred reduction, E°[Pt(bpy)/Pt(bpy)•−], is observed at −1.32 V, anodically shifted by 50–60 mV compared to that of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy). On the other hand, the first oxidation couple in the triads is found to be associated with the oxidation of the carbazole unit with the potential for oxidation of +1.36 V for (Cbz)2–Pt(bpy)–C60. The introduction of the di-tertbutyl group at the 3,6-positions of carbazole shifts the poten-
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tials to +1.24 V in (tBuCbz)2–Pt(bpy)–C60. The redox potentials of the investigated compounds are summarized in Table 1. The free energy changes for CS and CR processes in (Cbz)2– Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 are evaluated with the corrections of the Coulombic interactions between the centres of positive and negative charges using the Weller equations15 ΔGCR ¼ E°ðA=A• Þ E°ðD•þ =DÞ þ
e eV 4π ε0 εS RDA
ð1Þ
with the distances RPt–C60 = 9.80 Å, RD–Pt = 10.49 Å, RD–C60 (cis-) = 15.08 Å and RD–C60 (trans-) = 19.51 Å obtained from the optimized structure of (Cbz)2–Pt(bpy)–C60′. The calculated free energy changes for the CS and CR processes are listed in Table 2. The energy differences of the 3MLCT/LLCT excited state and the ground state (E00) are estimated from the intersection of the emission and excitation spectra of (Cbz)2– Pt(bpy) and (tBuCbz)2–Pt(bpy) measured at room temperature
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in deaerated dichloromethane, and are estimated to be 2.51 eV and 2.42 eV, respectively. Transient absorption spectroscopy
Fig. 1 UV-Visible absorption spectra of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 as well as their model compounds (Cbz)2– Pt(bpy) and (tBuCbz)2–Pt(bpy) in CH2Cl2.
Fig. 2
The photoinduced electron transfer reactions in (Cbz)2– Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 were studied by femtosecond transient absorption spectroscopy and the resulting spectra were subjected to global fitting with a sequential model16 to yield evolution-associated difference spectra (EADS) and with a parallel model to yield decay-associated difference spectra (DADS) (Fig. 4 and 5). (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 behave similarly upon photo-excitation at 400 nm in dichloromethane with the first resolved component depicting the formation of the 3MLCT/LLCT excited state (Fig. 4b and 5b). The 3MLCT/LLCT excited state is generated within the instrumental response time with a time constant of τ < 150 fs via ISC of the initially populated 1MLCT/ LLCT excited state upon laser irradiation. The first EADS
Spatial plots (isovalue = 0.03) of selected molecular orbitals of (Cbz)2–Pt(bpy)–C60’ obtained from PBE0/CPCM calculation.
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Fig. 3 Normalized emission spectra of (Cbz)2–Pt(bpy) and (tBuCbz)2– Pt(bpy) in deaerated CH2Cl2.
Table 1 Standard potentials (vs. SCE, in V)a of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, and their model compounds CbzC6H4–CuC– Ph, tBuCbzC6H4–CuC–Ph, (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) in CH2Cl2
E°/V (vs. SCE) in CH2Cl2 Molecule
D•+/D
C60/C60•−
C60•−/C602−
Pt(bpy)/ Pt(bpy)•−
CbzC6H4–CuC–Ph t BuCbzC6H4–CuC–Ph (Cbz)2–Pt(bpy) (tBuCbz)2–Pt(bpy) (Cbz)2–Pt(bpy)–C60 (tBuCbz)2–Pt(bpy)–C60
+1.35b +1.21 +1.33b +1.24 +1.36b +1.24
— — — — −0.59 −0.59
— — — — −0.98 −0.98
— — −1.37 −1.38 −1.32 −1.32
a
The standard potentials were determined as E° = (E°pa + E°pc)/2 by cyclic voltammetry in CH2Cl2 using n-Bu4NPF6 (0.1 M) as the supporting electrolyte; scan rate = 100 mV s−1. b Quasi-reversible couple.
shows a broad absorption between 550 and 700 nm, which is attributed to the 3MLCT excited state.12b,i A positive absorption at wavelengths shorter than 475 nm as well as a broad absorption in the near-infrared region (λ > 900 nm) are also observed.
These absorptions are different from the characteristic signal of Cbz•+ (λmax ∼ 750 nm).17 This may be due to the charge delocalization across the Pt–CuC–Ph and the Cbz/tBuCbz units in the 3LLCT excited state, (Cbz)2•+–Pt(bpy)•−–C60 and (tBuCbz)2•+–Pt(bpy)•−–C60 (see HOMO and HOMO−1 of (Cbz)2– Pt(bpy) in Fig. S1†), leading to absorptions quite similar to that observed for the cation radical of a Cbz dimer, (Cbz)2•+.17 Subsequent to the formation of the 3MLCT/LLCT excited state is the electron transfer reaction generating the fully CS species, namely (Cbz)2•+–Pt(bpy)–C60•− and (tBuCbz)2•+–Pt(bpy)–C60•−, with a rise time τ1 = 185 fs and 221 fs, respectively. The formation of the CS states is characterized by the strong transient absorption at ca. 1000 nm, which is the characteristic signal of C60•−.1a,3b,5e It should be noted that the formation of the CS states is very fast, much faster than that of the vibrational relaxation of the 3MLCT/LLCT excited state which was determined to be ∼4 ps.12i As a result, the photoinduced electron transfer reaction is expected to occur at a vibrationally unrelaxed 3MLCT/LLCT excited state,18 and the subsequent vibrational relaxation of the resulting CS state occurs at a time constant τ2 = 1.7 ps for (Cbz)2–Pt(bpy)–C60 and 1.5 ps for (tBuCbz)2–Pt(bpy)–C60. The lifetime of the CS states is determined to be τ3 = 56 ps for (Cbz)2–Pt(bpy)–C60 and τ3 = 331 ps for (tBuCbz)2–Pt(bpy)–C60. On the other hand, laser irradiation at 400 nm also populates the 1C60 excited state, where the extinction coefficient (ε) of N-methylfulleropyrrolidine at 400 nm is ca. 5000 dm3 mol−1 cm−1.3b Therefore, the first four resolved EADS components of both complexes depict the presence of the 1C60 excited state with absorption features at λmax = 510 and 890 nm.3b The 1C60* eventually proceeds to the 3 C60* through ISC at time constants of τ4 = 896 ps and τ4 = 943 ps for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, respectively. The final resolved component with strong transient absorption between 550 and 750 nm (λmax = 680 nm) is attributed to the 3C60* with a lifetime τ5 > 6 ns.3b,5e,19 In order to investigate reactions at times over 6 ns, nanosecond transient absorption spectroscopy was carried out on both triads and their observed absorption difference spectra taken at t = 5 μs after laser irradiation at 355 nm in dichloromethane are shown in Fig. 4d and 5d. These spectra match the EADS of the final component of both complexes with strong transient absorption between 550 and 750 nm (λmax = 680 nm), which
Table 2 Free energy changes ΔG (eV) for electron-transfer reactions in CH2Cl2 calculated from the electrochemical potentials after correction for the Coulomb interaction between the chargesa
Reaction
CS
CR
(Cbz)2–Pt(bpy)–C60 → (Cbz)2•+–Pt(bpy)•−–C60 (Cbz)2•+–Pt(bpy)•−–C60 → (Cbz)2•+–Pt(bpy)–C60•− (tBuCbz)2–Pt(bpy)–C60 → (tBuCbz)2•+–Pt(bpy)•−–C60 (tBuCbz)2•+–Pt(bpy)•−–C60 → (tBuCbz)2•+–Pt(bpy)–C60•−
— −0.69 (−0.66)b — −0.69 (−0.66)b
−2.53 −1.84 (−1.87)b −2.41 −1.72 (−1.75)b
a ΔGCR = E°(A/A•−) − E°(D•+/D) + e/4πε0εsRDA is for charge-recombination where RDA is the charge-separation distance determined from DFT; ΔGCS is the driving force for charge-separation. The parameters used are ε0 = 8.85, εs = 8.93 for CH2Cl2, RPt–C60 = 9.80 Å, RD–Pt = 10.49 Å, RD–C60 (cis-) = 15.08 Å, RD–C60 (trans-) = 19.51 Å with the distances taken from the DFT optimized structure of (Cbz)2–Pt(bpy)–C60′. b Driving forces for radical ion pairs in the trans-position.
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Fig. 4 Transient absorption difference spectra of (Cbz)2–Pt(bpy)–C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c) time decay profiles of optical density at 460 nm, 680 nm and 1020 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated CH2Cl2, with the time-decay profile of optical density at 690 nm (inset).
is the characteristic signal of the 3C60*.3b,5e,19 The 3C60* eventually decays to the ground state at time constants of τ = 28 and 22 μs, respectively, for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 (Fig. 4d and 5d), close to that found for a functionalized C60 of 25 μs.20 The energetic and kinetic data for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 are summarized in Scheme 2. It is noteworthy that the CS states, (Cbz)2•+–Pt(bpy)–C60•− and (tBuCbz)2•+–Pt(bpy)–C60•−, are produced via photoinduced electron transfer from the 3MLCT/LLCT excited state, and therefore are in the triplet manifold. As the CR processes occur in a few hundred picoseconds, CR should therefore proceed through a spin-allowed process to the lower-lying 3C60* state instead of a much slower spin-forbidden direct recombination process to the singlet ground state. In addition, with large driving forces (ΔG = −1.84 eV for (Cbz)2–Pt(bpy)–C60 and −1.72 eV for (tBuCbz)2–Pt(bpy)–C60) and the small reorganization energy in C60-containing complexes,14,15 CR processes are expected to be in the Marcus inverted region,21 and hence the direct recombination process to the ground state should be slow. The recombination to the lower-lying 3C60* state is further supported experimentally in that the diminishing of
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the CS transient absorption is accompanied by the rise of the C60* absorption at a similar rate (Fig. 4c and 5c).
3
Conclusions Donor–chromophore–acceptor triads, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, were synthesized and their photophysical and electrochemical properties were studied. The photoinduced electron transfer processes were investigated by femtosecond and nanosecond transient absorption spectroscopy. Laser irradiation at 400 nm predominantly excites the MLCT/LLCT transition, followed by an ultrafast ISC to generate the 3MLCT/LLCT excited state, from which CS occurs within a picosecond. The triplet CS state decays to the energetically lower-lying spin-allowed 3C60* state in a few hundred picoseconds instead of the spin-forbidden singlet ground state. The resulting long-lived 3C60* state decays to the ground state in the microsecond timescale. The ultrafast ISC in the triads through the strong spin–orbit coupling mediated by the platinum(II) centre and the subsequent ultrafast electron transfer, which eventually generates the CS state in the triplet
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Fig. 5 Transient absorption difference spectra of (tBuCbz)2–Pt(bpy)–C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c) time decay profiles of optical density at 460 nm, 680 nm and 1020 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated CH2Cl2, with the time-decay profile of optical density at 680 nm (inset).
manifold, establishes another strategy for the development of new donor–acceptor systems that are capable of effectively generating triplet CS states with long CS lifetimes.
Experimental section General
tions for photophysical studies were deaerated on a highvacuum line in a two-compartment cell that consisted of a 10 mL Pyrex bulb and a 1 cm path length quartz cuvette and sealed against the atmosphere with a Bibby Rotaflo HP6 Teflon stopper. The solutions were rigorously degassed with at least four successive freeze–pump–thaw cycles. The luminescence quantum yield was measured at room temperature using [Ru(bpy)3]Cl2 as a standard (Φem = 0.06).22
1
H NMR spectra were recorded on a Bruker AVANCE 300 (300 MHz) or Bruker AVANCE 400 (400 MHz) Fourier-transform NMR spectrometer with chemical shifts reported relative to tetramethylsilane (Me4Si). Positive-ion FAB mass spectra were recorded on a Thermo Scientific DFS high-resolution magnetic sector mass spectrometer. Elemental analyses of the newly synthesized complexes were performed using a Flash EA 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences, Beijing. Ultraviolet-visible spectra were recorded on a Varian Cary 50 UV-vis spectrophotometer in spectroscopic grade CH2Cl2. Steady-state emission spectra were recorded on a Spex Fluorolog-3 Model FL3-211 spectrofluorometer equipped with a R2658P PMT detector. All solu-
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Electrochemical measurements The cyclic voltammetry measurements were performed on a CHI 620A electrochemical analyzer in deaerated dichloromethane containing 0.10 M n-Bu4NPF6 as the supporting electrolyte at 298 K. A glassy carbon working electrode was polished with Linde polishing alumina suspension and rinsed with deionized water followed by dichloromethane before use. A platinum wire was used as a counter electrode and an Ag/ AgNO3 (0.1 M in acetonitrile) was used as a reference electrode. All experiments were followed by the addition of ferrocene and the ferrocene/ferricenium redox potential was used as an internal reference.
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by global analysis16 with the graphical interface program Glotaran23 (http://glotaran.org/). Nanosecond transient absorption spectroscopy experiments were performed using an LP920-KS Laser Flash Photolysis Spectrometer (Edinburgh Instruments Ltd, Livingston, UK) at ambient temperature. The excitation source was the 355 nm output (third harmonic) of an Nd:YAG laser (Spectra-Physics Quanta-Ray Lab-130 Pulsed Nd:YAG Laser) and the probe light source was a Xe900 450 W xenon arc lamp. The transient absorption spectra were recorded using an image-intensified CCD camera (Andor) with a PC plug-in controller, fully operated by the L900 spectrometer software. The absorption kinetics were determined using a Hamamatsu R928 photomultiplier tube and recorded using a Tektronix Model TDS3012B (100 MHz, 1.25 GS s−1) digital oscilloscope. Samples were freshly prepared and degassed with at least four freeze–pump–thaw cycles on a high-vacuum line in a two compartment cell that consisted of a 10 mL Pyrex bulb and a 1 cm path length quartz cuvette. Computational details
Scheme 2 Schematic energy diagram for photoinduced electron transfer of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 in CH2Cl2. aEnergies of 1C60* and 3C60* excited states are obtained from ref. 19a. b Energy for radical ion pairs in the trans-position.
Transient absorption (TA) measurements The fs time-resolved experiments were performed based on a commercial Ti:Sapphire regenerative amplifier laser system and home-built TA spectrometers. The sample solutions were excited using a 400 nm pump laser; the subsequent excited state processes taking place on the fs to several ns time-scale were detected by a second probe pulse induced by an 800 nm laser in the TA method. The 400 nm pump laser was the second harmonic of the 800 nm fundamental (1 kHz repetition rate, ∼35 fs duration) produced with a BBO crystal. In the fs-TA experiments, the excited states were probed by a white light continuum (WLC) pulse produced by focusing part of the 800 nm laser onto a rotating quartz plate. The TA signals were collected by a monochromator and detected with a thermoelectric cooled CCD detector. The spectral window for the TA covers 450–740 nm and 600–1050 nm regions. TA signals at wavelengths from 740 nm to 880 nm were masked by the 800 nm fundamental and thus were not shown in the fs-TA in Fig. 4 and 5. Temporal delay of the probe relative to the pump pulse was controlled by an optical delay line in the fs-TA system. The instrument response function (IRF) of the ultrafast system is wavelength-dependent. As the detection wavelength varies from 1050 to 450 nm, the IRF changes from ∼150 to 250 fs. For the fs-TA spectra, corrections have been made to eliminate the wavelength-dependent time shift caused by the group velocity dispersion. The fs-TA spectra were analyzed
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Calculations were carried out using the Gaussian 09 software package.24 The ground-state geometries of (Cbz)2–Pt(bpy) and (Cbz)2–Pt(bpy)–C60′ were fully optimized by the density functional theory (DFT) with the hybrid Perdew, Burke, and Ernzerhof functional (PBE0).25 Vibrational frequency calculations were performed for all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface. On the basis of the ground state optimized geometries, the timedependent density functional theory (TDDFT)26 calculation at the same level of theory associated with the conductor-like polarizable continuum model (CPCM)27 using CH2Cl2 as the solvent was employed to compute the singlet–singlet transitions for the complexes. For all the calculations, the Stuttgart effective core potentials (ECPs) and the associated basis set were applied to describe Pt28 with f-type polarization functions (ζ = 0.993),29 while the 6-31G(d,p) basis set30 was used for all other atoms. The DFT and TDDFT calculations were performed with a pruned (99 590) grid. Synthesis 3,6-Di-tert-butylcarbazole,31 N-(4-bromophenyl)carbazole,32 N-(4-bromophenyl)-3,6-di-tert-butylcarbazole,32 N-(4-ethynylphenyl)carbazole,33 N-(4-ethynylphenyl)-3,6-di-tert-butylcarbazole,33 9-(4-(2-phenylethynyl)phenyl)carbazole (CbzC6H4– CuC–Ph),34 3,6-di-tert-butyl-9-(4-(2-phenylethynyl)phenyl)carbazole (tBuCbzC6H4–CuC–Ph)34 and fullerene-appended bipyridine13 were prepared according to literature procedures. General procedure for platinum(II) complexation 4,4′-Dimethyl-2,2′-bipyridine/fullerene-appended bipyridine and [PtCl2(DMSO)2] were stirred in dichloromethane under N2 for 15 h. The solvent was then reduced in volume and diethyl ether was added. The resulting precipitate, dichloroplatinum(II) precursor, was filtered and dried. Dichloroplatinum(II) precursor and arylacetylene (2.5 equiv.) were added to deaerated
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dichloromethane. Triethylamine (∼1 mL) was added, followed by the addition of copper iodide (∼2 mg). The reaction mixture was stirred at room temperature under N2 for 15 h. The reaction was filtered and the solvent was removed under reduced pressure. The product was purified by column chromatography on silica gel using chloroform–1% methanol as the eluent, and then recrystallized from chloroform–diethyl ether or chloroform–methanol. (Cbz)2–Pt(bpy). Yield: 68%. 1H NMR (400 MHz, CDCl3): δ/ppm 9.64 (d, J = 5.7 Hz, 2H), 8.14 (d, J = 7.7 Hz, 4H), 7.92 (s, 2H), 7.76 (d, J = 8.5 Hz, 4H), 7.43 (m, 14H), 7.28 (m, 4H), 2.55 (s, 6H). FAB-MS (m/z): [M + H]+ 912.2. Elemental analysis calcd (%) for C52H36N4Pt·2H2O: C 65.88, H 4.25, N 5.91; found: C 66.18, H 3.95, N 6.04. (tBuCbz)2–Pt(bpy). Yield: 62%. 1H NMR (400 MHz, CDCl3): δ/ppm 9.57 (d, J = 5.6 Hz, 2H), 8.13 (d, J = 1.3 Hz, 4H), 7.96 (s, 2H), 7.73 (d, J = 8.5 Hz, 4H), 7.48 (d, J = 1.6 Hz, 2H), 7.45 (d, J = 8.5 Hz, 6H), 7.38 (d, J = 8.5 Hz, 6H), 2.55 (s, 6H), 1.47 (s, 36H). FAB-MS (m/z): [M]+ 1135.4. Elemental analysis calcd (%) for C68H68N4Pt·H2O: C 70.75, H 6.11, N 4.85; found: C 70.79, H 6.02, N 4.81. (Cbz)2–Pt(bpy)–C60. Yield: 39%. 1H NMR (400 MHz, C2D4Cl2, 75 °C): δ/ppm 9.93 (d, J = 5.7 Hz, 1H), 9.67 (d, J = 5.7 Hz, 1H), 8.69 (s, 1H), 8.19 (d, J = 6.3 Hz, 1H), 8.12 (d, J = 7.7 Hz, 4H), 8.09 (s, 1H), 7.66 (t, J = 7.9 Hz, 4H), 7.45 (d, J = 8.3 Hz, 4H), 7.40 (m, 9H), 7.26 (m, 4H), 5.43 (s, 1H), 5.20 (d, J = 9.6 Hz, 1H), 4.26 (d, J = 9.6 Hz, 1H), 3.19 (m, 1H), 2.78 (m, 1H), 2.53 (s, 3H), 2.51 (m, 1H), 2.01 (m, 1H), 1.92 (m, 1H), 1.72 (m, 1H), 1.58 (m, 1H), 1.46 (m, 3H), 1.29 (bs, 12H), 0.88 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): [M − H]+ 1826.6. Elemental analysis calcd (%) for C125H61N5Pt·3/2CH3OH·1/2CHCl3: C 78.80, H 3.51, N 3.62; found: C 78.65, H 3.71, N 3.99. (tBuCbz)2–Pt(bpy)–C60. (Yield: 33%). 1H NMR (400 MHz, C2D2Cl4, 90 °C): δ/ppm 10.01 (d, J = 4.1 Hz, 1H), 9.79 (d, J = 4.1 Hz, 1H), 8.58 (s, 1H), 8.20 (d, J = 5.5 Hz, 1H), 8.15 (s, 4H), 7.95 (s, 1H), 7.75 (t, J = 7.5 Hz, 4H), 7.51 (d, J = 7.5 Hz, 9H), 7.42 (d, J = 6.8, 4H), 5.34 (s, 1H), 5.26 (d, J = 9.6 Hz, 1H), 4.36 (d, J = 9.6 Hz, 1H), 3.24 (m, 1H), 2.83 (m, 1H), 2.60 (s, 3H), 2.55 (m, 1H), 2.09 (m, 1H), 2.01 (m, 1H), 1.77 (m, 1H), 1.67 (m, 1H), 1.50 (s, 39H), 1.34 (bs, 12H), 0.94 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): [M − 2H]+ 2049.5. Elemental analysis calcd (%) for C141H93N5Pt·CH3OH·1/2CHCl3: C 79.83, H 4.58, N 3.27; found: C 79.89, H 4.95, N 3.55.
Acknowledgements V.W.-W.Y. acknowledges support from The University of Hong Kong and the URC Strategic Research Theme on New Materials. This work was supported by a grant from the Theme-Based Research Scheme (TBRS) of the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. T23-713/11). We are grateful to the Information Technology Services of The University of Hong Kong for providing computational resources.
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Cite this: Dalton Trans., 2014, 43, 17624
Synthesis and photoinduced electron transfer in platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes† Sai-Ho Lee,a Chris Tsz-Leung Chan,b Keith Man-Chung Wong,a Wai Han Lam,a Wai-Ming Kwokb and Vivian Wing-Wah Yam*a Platinum(II) bis(N-(4-ethynylphenyl)carbazole)bipyridine fullerene complexes, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, were synthesized. Their photophysical properties were studied by electronic absorption and emission spectroscopy and the origin of the transitions was supported by computational studies. The electrochemical properties were also studied and the free energies for charge-separation and charge-recombination processes were evaluated. The photoinduced electron transfer reactions in the triads were investigated by femtosecond and nanosecond transient absorption spectroscopy. In
Received 12th May 2014, Accepted 8th August 2014 DOI: 10.1039/c4dt01397b www.rsc.org/dalton
dichloromethane, both triads undergo ultrafast charge separation from the 3MLCT/LLCT excited state within 300 fs to yield their respective triplet charge-separated (CS) states, namely (Cbz)2•+–Pt(bpy)–C60•− and (tBuCbz)2•+–Pt(bpy)–C60•−, and the CS states would undergo charge recombination to give the 3C60* state, which subsequently decays to the ground state in 22–28 μs.
Introduction The study of photoinduced electron transfer reactions in donor–acceptor systems has been an active area of research owing to their potential applications in artificial photosynthesis and photovoltaic devices.1 Donor–acceptor systems, particularly those utilizing fullerene as an electron acceptor, have attracted considerable attention in recent years.2 Fullerene is known to be an excellent electron acceptor due to its symmetrical shape, large size, and π-electron delocalization over the three-dimensional surface.2b,3 With small reorganization energy, efficient charge separation and slow charge recombination were accomplished by utilizing C60 as an electron acceptor. On the other hand, carbazole (Cbz), an aromatic amine, and its derivatives are known to be good electron donors4 and have been employed to develop a number of Cbz–C60 donor– acceptor systems with their photoinduced intramolecular electron transfer reactions well investigated.5 It has been demonstrated that the lifetime of the triplet charge-separated (CS) state in a Cbz–C60 dyad generated via 3C60* is significantly a Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong, P.R. China b Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, P.R. China. E-mail: [email protected] † Electronic supplementary information (ESI) available: The selected singlet excited states (Sn) computed by TDDFT/CPCM calculation, selected molecular orbitals and Cartesian coordinates of the optimized structures. See DOI: 10.1039/c4dt01397b
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longer than that of the singlet CS state generated via 1C60*,5b as charge recombination (CR) from the triplet CS state to the singlet ground state is a spin-forbidden process. One effective approach to develop Cbz–C60 systems that are capable of generating the triplet CS state would be to utilize transition metalbased chromophores for the development of donor–chromophore–acceptor triads. Transition metal-based chromophores which have low lying metal-to-ligand charge-transfer (MLCT) excited states, such as Ru(II),6 Re(I),7 Os(II),8 Ir(III),1b,6a,b,8g,9 Cu(I)10 and Pt(II),11 have been extensively studied. Owing to the ultrafast intersystem crossing (ISC) through the strong spin– orbit coupling imparted by the heavy metal centres, charge transfer processes generally occur at the 3MLCT excited state, and hence the spin-multiplicity of the CS state can be controlled. Unlike Ru(II),6 Re(I),7 Os(II),8 Ir(III),1b,6a,b,8g,9 and Cu(I),10 square-planar d8 Pt(II) based donor–acceptor systems are relatively less extensively studied. It was only until quite recently that Pt(II) based donor–acceptor systems have been explored for their photoinduced electron transfer properties,11 mainly encouraged by the development of the Pt(II) polypyridine alkynyl chromophores that display enhanced 3MLCT excited state behavior through the enlargement of the d–d orbital splitting by the incorporation of an alkynyl ligand that leads to a higher-lying d–d ligand-field state.12 In the present study, we develop a series of donor–chromophore–acceptor systems based on the bipyridine Pt(II) bisalkynyl chromophore. Herein, we report the synthesis, electrochemical and photophysical
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Electronic absorption and emission spectroscopy
Chart 1
studies of a series of donor–chromophore–acceptor triads (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 as well as their relevant model compounds, (Cbz)2–Pt(bpy), (tBuCbz)2–Pt(bpy), CbzC6H4–CuC–Ph and tBuCbzC6H4–CuC–Ph (Chart 1), using a variety of experimental techniques including electronic absorption and emission spectroscopy, cyclic voltammetry, femtosecond and nanosecond time-resolved transient absorption spectroscopy, and the photochemical processes are identified and the rate constants are determined. Density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations were performed to provide further insights into the origin of the electronic absorption and emission properties.
Results and discussion Synthesis The synthesis of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 is shown in Scheme 1. N-(4-Ethynylphenyl)carbazole and N(4-ethynylphenyl)-3,6-di-tert-butyl-carbazole were first prepared by the Ullmann-type reaction between carbazoles and 1-bromo-4-iodobenzene, followed by the palladium-catalyzed Sonagashira coupling with (trimethylsilyl)acetylene (TMSA), and the subsequent removal of the TMS group with potassium carbonate in tetrahydrofuran–methanol solution. On the other hand, according to literature procedures,13 the fullereneappended bipyridine derivative was obtained by the oxidation of 4,4′-dimethyl-2,2′-bipyridine with selenium oxide yielding 4-methyl-2,2′-bipyridine-4′-carboxaldehyde, followed by the incorporation of fullerene with the Prato reaction.14 The fullerene-appended bipyridine derivative was reacted with [PtCl2(DMSO)2] to afford the dichloroplatinum(II) precursor, which was then reacted with alkynes using copper-catalyzed coupling reaction to yield (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– Pt(bpy)–C60, with their identities confirmed by 1H NMR spectroscopy, FAB mass spectrometry and elemental analysis.
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The electronic absorption spectra of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) exhibit an intense absorption band at ca. 280–360 nm and a moderately intense band at ca. 360–450 nm with a λmax at ca. 400 nm in dichloromethane (Fig. 1). The high-energy intense absorptions are the π–π* transitions of the carbazole, bipyridine and alkynyl moieties, whereas the less intense absorptions in the visible region are attributed to an admixture of platinum-to-bipyridine metal-to-ligand charge transfer (MLCT) and alkynyl-to-bipyridine ligand-to-ligand charge transfer (LLCT) transitions.12b,i,j With the C60 moiety appended in (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, an absorption tail extending to 700 nm, which can also be observed in the electronic absorption spectrum of N-methylfulleropyrrolidine,3b is tentatively assigned to the π–π* transitions of C60. TDDFT calculations were performed on selected complexes (Cbz)2–Pt(bpy) and (Cbz)2–Pt(bpy)–C60 to provide further insight into the nature of the electronic transitions for the complexes. To reduce the computational cost, the dodecyl group of N-dodecylpyrrolidine on C60 for (Cbz)2–Pt(bpy)–C60 was replaced by a methyl group and was labeled as (Cbz)2–Pt(bpy)–C60′ (see Computational details). Selected singlet–singlet transitions for the complexes are listed in Table S1† and the molecular orbitals involved in the transitions are shown in Fig. 2 and S1.† The first two transitions computed at 484 and 451 nm for (Cbz)2–Pt(bpy) are predominantly the LLCT [π(CuC–Ph–Cbz)→π*(bpy)] transitions, slightly mixed with the MLCT[dπ(Pt)→π*(bpy)] character (Table S1†). For (Cbz)2–Pt(bpy)–C60′, the several transitions computed to be less intense in the lower-energy region are the IL π–π* transitions of the C60. The LLCT [π(CuC–Ph–Cbz)→π*(C60)] excited state is the S2 state which is close in energy to the lowest-energy IL(C60) state. When compared with (Cbz)2–Pt(bpy), the LLCT [π(CuC–Ph–Cbz)→π*(bpy)]/MLCT[dπ(Pt)→π*(bpy)] transitions for (Cbz)2–Pt(bpy)–C60′ are found to be red-shifted. On photoexcitation, (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) display observable emission in dichloromethane (Fig. 3), while fullerene-appended complexes (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– Pt(bpy)–C60 are non-emissive under identical conditions. The emission is red-shifted after the introduction of the di-tertbutyl groups at the 3,6-positions of carbazole in (tBuCbz)2–Pt(bpy). The emission of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) was found to be strongly quenched with their luminescence quantum yields determined to be 0.03 and 0.01, respectively, much lower than 0.50 for a platinum(II) diimine bis(alkynyl) chromophore.12a The weak emissive nature of the carbazolecontaining complexes may possibly be due to the occurrence of a quenching process in the excited state. A possible interpretation of this is that a competitive process of photoinduced electron transfer between the carbazole moiety and the Pt(II) bipyridine chromophore that leads to photochemical charge separation has taken place. Similarly, the nonemissive nature of the fullerene-containing complexes could again be ascribed to photoinduced electron transfer processes.
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Scheme 1 Synthesis of donor–chromophore–acceptor triads, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60. Reagents and conditions: (i) 4-bromoiodobenzene, K2CO3, 18-crown-6, Cu, DMF, reflux; (ii) PdCl2(PPh3)2, CuI, PPh3, TMSA, Et3N, reflux; (iii) K2CO3, MeOH–THF, r.t.; (iv) SeO2, dioxane, reflux; (v) N-dodecyl glycine, C60, PhMe, reflux; (vi) PtCl2(DMSO)2, CH2Cl2, r.t.; (vii) CuI, Et3N, CH2Cl2, r.t.
Electrochemistry In order to evaluate the free energy changes associated with the CS and CR processes in (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2– Pt(bpy)–C60, cyclic voltammetry was carried out in 0.1 M n-Bu4NPF6 solution in deaerated dichloromethane. The first and second reduction couples in the triads are assigned to the reduction of the C60 moiety, with the observed reduction potentials E°(C60/C60•−) and E°(C60•−/C602−), at −0.59 V and −0.98 V (vs. SCE). Furthermore, the third reduction couple, which is assigned as the bipyridine ligand-centred reduction, E°[Pt(bpy)/Pt(bpy)•−], is observed at −1.32 V, anodically shifted by 50–60 mV compared to that of (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy). On the other hand, the first oxidation couple in the triads is found to be associated with the oxidation of the carbazole unit with the potential for oxidation of +1.36 V for (Cbz)2–Pt(bpy)–C60. The introduction of the di-tertbutyl group at the 3,6-positions of carbazole shifts the poten-
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tials to +1.24 V in (tBuCbz)2–Pt(bpy)–C60. The redox potentials of the investigated compounds are summarized in Table 1. The free energy changes for CS and CR processes in (Cbz)2– Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 are evaluated with the corrections of the Coulombic interactions between the centres of positive and negative charges using the Weller equations15 ΔGCR ¼ E°ðA=A• Þ E°ðD•þ =DÞ þ
e eV 4π ε0 εS RDA
ð1Þ
with the distances RPt–C60 = 9.80 Å, RD–Pt = 10.49 Å, RD–C60 (cis-) = 15.08 Å and RD–C60 (trans-) = 19.51 Å obtained from the optimized structure of (Cbz)2–Pt(bpy)–C60′. The calculated free energy changes for the CS and CR processes are listed in Table 2. The energy differences of the 3MLCT/LLCT excited state and the ground state (E00) are estimated from the intersection of the emission and excitation spectra of (Cbz)2– Pt(bpy) and (tBuCbz)2–Pt(bpy) measured at room temperature
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in deaerated dichloromethane, and are estimated to be 2.51 eV and 2.42 eV, respectively. Transient absorption spectroscopy
Fig. 1 UV-Visible absorption spectra of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 as well as their model compounds (Cbz)2– Pt(bpy) and (tBuCbz)2–Pt(bpy) in CH2Cl2.
Fig. 2
The photoinduced electron transfer reactions in (Cbz)2– Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 were studied by femtosecond transient absorption spectroscopy and the resulting spectra were subjected to global fitting with a sequential model16 to yield evolution-associated difference spectra (EADS) and with a parallel model to yield decay-associated difference spectra (DADS) (Fig. 4 and 5). (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 behave similarly upon photo-excitation at 400 nm in dichloromethane with the first resolved component depicting the formation of the 3MLCT/LLCT excited state (Fig. 4b and 5b). The 3MLCT/LLCT excited state is generated within the instrumental response time with a time constant of τ < 150 fs via ISC of the initially populated 1MLCT/ LLCT excited state upon laser irradiation. The first EADS
Spatial plots (isovalue = 0.03) of selected molecular orbitals of (Cbz)2–Pt(bpy)–C60’ obtained from PBE0/CPCM calculation.
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Fig. 3 Normalized emission spectra of (Cbz)2–Pt(bpy) and (tBuCbz)2– Pt(bpy) in deaerated CH2Cl2.
Table 1 Standard potentials (vs. SCE, in V)a of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, and their model compounds CbzC6H4–CuC– Ph, tBuCbzC6H4–CuC–Ph, (Cbz)2–Pt(bpy) and (tBuCbz)2–Pt(bpy) in CH2Cl2
E°/V (vs. SCE) in CH2Cl2 Molecule
D•+/D
C60/C60•−
C60•−/C602−
Pt(bpy)/ Pt(bpy)•−
CbzC6H4–CuC–Ph t BuCbzC6H4–CuC–Ph (Cbz)2–Pt(bpy) (tBuCbz)2–Pt(bpy) (Cbz)2–Pt(bpy)–C60 (tBuCbz)2–Pt(bpy)–C60
+1.35b +1.21 +1.33b +1.24 +1.36b +1.24
— — — — −0.59 −0.59
— — — — −0.98 −0.98
— — −1.37 −1.38 −1.32 −1.32
a
The standard potentials were determined as E° = (E°pa + E°pc)/2 by cyclic voltammetry in CH2Cl2 using n-Bu4NPF6 (0.1 M) as the supporting electrolyte; scan rate = 100 mV s−1. b Quasi-reversible couple.
shows a broad absorption between 550 and 700 nm, which is attributed to the 3MLCT excited state.12b,i A positive absorption at wavelengths shorter than 475 nm as well as a broad absorption in the near-infrared region (λ > 900 nm) are also observed.
These absorptions are different from the characteristic signal of Cbz•+ (λmax ∼ 750 nm).17 This may be due to the charge delocalization across the Pt–CuC–Ph and the Cbz/tBuCbz units in the 3LLCT excited state, (Cbz)2•+–Pt(bpy)•−–C60 and (tBuCbz)2•+–Pt(bpy)•−–C60 (see HOMO and HOMO−1 of (Cbz)2– Pt(bpy) in Fig. S1†), leading to absorptions quite similar to that observed for the cation radical of a Cbz dimer, (Cbz)2•+.17 Subsequent to the formation of the 3MLCT/LLCT excited state is the electron transfer reaction generating the fully CS species, namely (Cbz)2•+–Pt(bpy)–C60•− and (tBuCbz)2•+–Pt(bpy)–C60•−, with a rise time τ1 = 185 fs and 221 fs, respectively. The formation of the CS states is characterized by the strong transient absorption at ca. 1000 nm, which is the characteristic signal of C60•−.1a,3b,5e It should be noted that the formation of the CS states is very fast, much faster than that of the vibrational relaxation of the 3MLCT/LLCT excited state which was determined to be ∼4 ps.12i As a result, the photoinduced electron transfer reaction is expected to occur at a vibrationally unrelaxed 3MLCT/LLCT excited state,18 and the subsequent vibrational relaxation of the resulting CS state occurs at a time constant τ2 = 1.7 ps for (Cbz)2–Pt(bpy)–C60 and 1.5 ps for (tBuCbz)2–Pt(bpy)–C60. The lifetime of the CS states is determined to be τ3 = 56 ps for (Cbz)2–Pt(bpy)–C60 and τ3 = 331 ps for (tBuCbz)2–Pt(bpy)–C60. On the other hand, laser irradiation at 400 nm also populates the 1C60 excited state, where the extinction coefficient (ε) of N-methylfulleropyrrolidine at 400 nm is ca. 5000 dm3 mol−1 cm−1.3b Therefore, the first four resolved EADS components of both complexes depict the presence of the 1C60 excited state with absorption features at λmax = 510 and 890 nm.3b The 1C60* eventually proceeds to the 3 C60* through ISC at time constants of τ4 = 896 ps and τ4 = 943 ps for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, respectively. The final resolved component with strong transient absorption between 550 and 750 nm (λmax = 680 nm) is attributed to the 3C60* with a lifetime τ5 > 6 ns.3b,5e,19 In order to investigate reactions at times over 6 ns, nanosecond transient absorption spectroscopy was carried out on both triads and their observed absorption difference spectra taken at t = 5 μs after laser irradiation at 355 nm in dichloromethane are shown in Fig. 4d and 5d. These spectra match the EADS of the final component of both complexes with strong transient absorption between 550 and 750 nm (λmax = 680 nm), which
Table 2 Free energy changes ΔG (eV) for electron-transfer reactions in CH2Cl2 calculated from the electrochemical potentials after correction for the Coulomb interaction between the chargesa
Reaction
CS
CR
(Cbz)2–Pt(bpy)–C60 → (Cbz)2•+–Pt(bpy)•−–C60 (Cbz)2•+–Pt(bpy)•−–C60 → (Cbz)2•+–Pt(bpy)–C60•− (tBuCbz)2–Pt(bpy)–C60 → (tBuCbz)2•+–Pt(bpy)•−–C60 (tBuCbz)2•+–Pt(bpy)•−–C60 → (tBuCbz)2•+–Pt(bpy)–C60•−
— −0.69 (−0.66)b — −0.69 (−0.66)b
−2.53 −1.84 (−1.87)b −2.41 −1.72 (−1.75)b
a ΔGCR = E°(A/A•−) − E°(D•+/D) + e/4πε0εsRDA is for charge-recombination where RDA is the charge-separation distance determined from DFT; ΔGCS is the driving force for charge-separation. The parameters used are ε0 = 8.85, εs = 8.93 for CH2Cl2, RPt–C60 = 9.80 Å, RD–Pt = 10.49 Å, RD–C60 (cis-) = 15.08 Å, RD–C60 (trans-) = 19.51 Å with the distances taken from the DFT optimized structure of (Cbz)2–Pt(bpy)–C60′. b Driving forces for radical ion pairs in the trans-position.
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Fig. 4 Transient absorption difference spectra of (Cbz)2–Pt(bpy)–C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c) time decay profiles of optical density at 460 nm, 680 nm and 1020 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated CH2Cl2, with the time-decay profile of optical density at 690 nm (inset).
is the characteristic signal of the 3C60*.3b,5e,19 The 3C60* eventually decays to the ground state at time constants of τ = 28 and 22 μs, respectively, for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 (Fig. 4d and 5d), close to that found for a functionalized C60 of 25 μs.20 The energetic and kinetic data for (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 are summarized in Scheme 2. It is noteworthy that the CS states, (Cbz)2•+–Pt(bpy)–C60•− and (tBuCbz)2•+–Pt(bpy)–C60•−, are produced via photoinduced electron transfer from the 3MLCT/LLCT excited state, and therefore are in the triplet manifold. As the CR processes occur in a few hundred picoseconds, CR should therefore proceed through a spin-allowed process to the lower-lying 3C60* state instead of a much slower spin-forbidden direct recombination process to the singlet ground state. In addition, with large driving forces (ΔG = −1.84 eV for (Cbz)2–Pt(bpy)–C60 and −1.72 eV for (tBuCbz)2–Pt(bpy)–C60) and the small reorganization energy in C60-containing complexes,14,15 CR processes are expected to be in the Marcus inverted region,21 and hence the direct recombination process to the ground state should be slow. The recombination to the lower-lying 3C60* state is further supported experimentally in that the diminishing of
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the CS transient absorption is accompanied by the rise of the C60* absorption at a similar rate (Fig. 4c and 5c).
3
Conclusions Donor–chromophore–acceptor triads, (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60, were synthesized and their photophysical and electrochemical properties were studied. The photoinduced electron transfer processes were investigated by femtosecond and nanosecond transient absorption spectroscopy. Laser irradiation at 400 nm predominantly excites the MLCT/LLCT transition, followed by an ultrafast ISC to generate the 3MLCT/LLCT excited state, from which CS occurs within a picosecond. The triplet CS state decays to the energetically lower-lying spin-allowed 3C60* state in a few hundred picoseconds instead of the spin-forbidden singlet ground state. The resulting long-lived 3C60* state decays to the ground state in the microsecond timescale. The ultrafast ISC in the triads through the strong spin–orbit coupling mediated by the platinum(II) centre and the subsequent ultrafast electron transfer, which eventually generates the CS state in the triplet
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Fig. 5 Transient absorption difference spectra of (tBuCbz)2–Pt(bpy)–C60 following excitation at 400 nm in CH2Cl2: (a) contour plot of the spectroscopic responses following femtosecond laser pulse irradiation; (b) EADS (top) and DADS (bottom) derived from the global analysis; (c) time decay profiles of optical density at 460 nm, 680 nm and 1020 nm; and (d) taken at 5 μs after nanosecond laser pulse excitation at 355 nm in deaerated CH2Cl2, with the time-decay profile of optical density at 680 nm (inset).
manifold, establishes another strategy for the development of new donor–acceptor systems that are capable of effectively generating triplet CS states with long CS lifetimes.
Experimental section General
tions for photophysical studies were deaerated on a highvacuum line in a two-compartment cell that consisted of a 10 mL Pyrex bulb and a 1 cm path length quartz cuvette and sealed against the atmosphere with a Bibby Rotaflo HP6 Teflon stopper. The solutions were rigorously degassed with at least four successive freeze–pump–thaw cycles. The luminescence quantum yield was measured at room temperature using [Ru(bpy)3]Cl2 as a standard (Φem = 0.06).22
1
H NMR spectra were recorded on a Bruker AVANCE 300 (300 MHz) or Bruker AVANCE 400 (400 MHz) Fourier-transform NMR spectrometer with chemical shifts reported relative to tetramethylsilane (Me4Si). Positive-ion FAB mass spectra were recorded on a Thermo Scientific DFS high-resolution magnetic sector mass spectrometer. Elemental analyses of the newly synthesized complexes were performed using a Flash EA 1112 elemental analyzer at the Institute of Chemistry, Chinese Academy of Sciences, Beijing. Ultraviolet-visible spectra were recorded on a Varian Cary 50 UV-vis spectrophotometer in spectroscopic grade CH2Cl2. Steady-state emission spectra were recorded on a Spex Fluorolog-3 Model FL3-211 spectrofluorometer equipped with a R2658P PMT detector. All solu-
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Electrochemical measurements The cyclic voltammetry measurements were performed on a CHI 620A electrochemical analyzer in deaerated dichloromethane containing 0.10 M n-Bu4NPF6 as the supporting electrolyte at 298 K. A glassy carbon working electrode was polished with Linde polishing alumina suspension and rinsed with deionized water followed by dichloromethane before use. A platinum wire was used as a counter electrode and an Ag/ AgNO3 (0.1 M in acetonitrile) was used as a reference electrode. All experiments were followed by the addition of ferrocene and the ferrocene/ferricenium redox potential was used as an internal reference.
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by global analysis16 with the graphical interface program Glotaran23 (http://glotaran.org/). Nanosecond transient absorption spectroscopy experiments were performed using an LP920-KS Laser Flash Photolysis Spectrometer (Edinburgh Instruments Ltd, Livingston, UK) at ambient temperature. The excitation source was the 355 nm output (third harmonic) of an Nd:YAG laser (Spectra-Physics Quanta-Ray Lab-130 Pulsed Nd:YAG Laser) and the probe light source was a Xe900 450 W xenon arc lamp. The transient absorption spectra were recorded using an image-intensified CCD camera (Andor) with a PC plug-in controller, fully operated by the L900 spectrometer software. The absorption kinetics were determined using a Hamamatsu R928 photomultiplier tube and recorded using a Tektronix Model TDS3012B (100 MHz, 1.25 GS s−1) digital oscilloscope. Samples were freshly prepared and degassed with at least four freeze–pump–thaw cycles on a high-vacuum line in a two compartment cell that consisted of a 10 mL Pyrex bulb and a 1 cm path length quartz cuvette. Computational details
Scheme 2 Schematic energy diagram for photoinduced electron transfer of (Cbz)2–Pt(bpy)–C60 and (tBuCbz)2–Pt(bpy)–C60 in CH2Cl2. aEnergies of 1C60* and 3C60* excited states are obtained from ref. 19a. b Energy for radical ion pairs in the trans-position.
Transient absorption (TA) measurements The fs time-resolved experiments were performed based on a commercial Ti:Sapphire regenerative amplifier laser system and home-built TA spectrometers. The sample solutions were excited using a 400 nm pump laser; the subsequent excited state processes taking place on the fs to several ns time-scale were detected by a second probe pulse induced by an 800 nm laser in the TA method. The 400 nm pump laser was the second harmonic of the 800 nm fundamental (1 kHz repetition rate, ∼35 fs duration) produced with a BBO crystal. In the fs-TA experiments, the excited states were probed by a white light continuum (WLC) pulse produced by focusing part of the 800 nm laser onto a rotating quartz plate. The TA signals were collected by a monochromator and detected with a thermoelectric cooled CCD detector. The spectral window for the TA covers 450–740 nm and 600–1050 nm regions. TA signals at wavelengths from 740 nm to 880 nm were masked by the 800 nm fundamental and thus were not shown in the fs-TA in Fig. 4 and 5. Temporal delay of the probe relative to the pump pulse was controlled by an optical delay line in the fs-TA system. The instrument response function (IRF) of the ultrafast system is wavelength-dependent. As the detection wavelength varies from 1050 to 450 nm, the IRF changes from ∼150 to 250 fs. For the fs-TA spectra, corrections have been made to eliminate the wavelength-dependent time shift caused by the group velocity dispersion. The fs-TA spectra were analyzed
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Calculations were carried out using the Gaussian 09 software package.24 The ground-state geometries of (Cbz)2–Pt(bpy) and (Cbz)2–Pt(bpy)–C60′ were fully optimized by the density functional theory (DFT) with the hybrid Perdew, Burke, and Ernzerhof functional (PBE0).25 Vibrational frequency calculations were performed for all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface. On the basis of the ground state optimized geometries, the timedependent density functional theory (TDDFT)26 calculation at the same level of theory associated with the conductor-like polarizable continuum model (CPCM)27 using CH2Cl2 as the solvent was employed to compute the singlet–singlet transitions for the complexes. For all the calculations, the Stuttgart effective core potentials (ECPs) and the associated basis set were applied to describe Pt28 with f-type polarization functions (ζ = 0.993),29 while the 6-31G(d,p) basis set30 was used for all other atoms. The DFT and TDDFT calculations were performed with a pruned (99 590) grid. Synthesis 3,6-Di-tert-butylcarbazole,31 N-(4-bromophenyl)carbazole,32 N-(4-bromophenyl)-3,6-di-tert-butylcarbazole,32 N-(4-ethynylphenyl)carbazole,33 N-(4-ethynylphenyl)-3,6-di-tert-butylcarbazole,33 9-(4-(2-phenylethynyl)phenyl)carbazole (CbzC6H4– CuC–Ph),34 3,6-di-tert-butyl-9-(4-(2-phenylethynyl)phenyl)carbazole (tBuCbzC6H4–CuC–Ph)34 and fullerene-appended bipyridine13 were prepared according to literature procedures. General procedure for platinum(II) complexation 4,4′-Dimethyl-2,2′-bipyridine/fullerene-appended bipyridine and [PtCl2(DMSO)2] were stirred in dichloromethane under N2 for 15 h. The solvent was then reduced in volume and diethyl ether was added. The resulting precipitate, dichloroplatinum(II) precursor, was filtered and dried. Dichloroplatinum(II) precursor and arylacetylene (2.5 equiv.) were added to deaerated
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dichloromethane. Triethylamine (∼1 mL) was added, followed by the addition of copper iodide (∼2 mg). The reaction mixture was stirred at room temperature under N2 for 15 h. The reaction was filtered and the solvent was removed under reduced pressure. The product was purified by column chromatography on silica gel using chloroform–1% methanol as the eluent, and then recrystallized from chloroform–diethyl ether or chloroform–methanol. (Cbz)2–Pt(bpy). Yield: 68%. 1H NMR (400 MHz, CDCl3): δ/ppm 9.64 (d, J = 5.7 Hz, 2H), 8.14 (d, J = 7.7 Hz, 4H), 7.92 (s, 2H), 7.76 (d, J = 8.5 Hz, 4H), 7.43 (m, 14H), 7.28 (m, 4H), 2.55 (s, 6H). FAB-MS (m/z): [M + H]+ 912.2. Elemental analysis calcd (%) for C52H36N4Pt·2H2O: C 65.88, H 4.25, N 5.91; found: C 66.18, H 3.95, N 6.04. (tBuCbz)2–Pt(bpy). Yield: 62%. 1H NMR (400 MHz, CDCl3): δ/ppm 9.57 (d, J = 5.6 Hz, 2H), 8.13 (d, J = 1.3 Hz, 4H), 7.96 (s, 2H), 7.73 (d, J = 8.5 Hz, 4H), 7.48 (d, J = 1.6 Hz, 2H), 7.45 (d, J = 8.5 Hz, 6H), 7.38 (d, J = 8.5 Hz, 6H), 2.55 (s, 6H), 1.47 (s, 36H). FAB-MS (m/z): [M]+ 1135.4. Elemental analysis calcd (%) for C68H68N4Pt·H2O: C 70.75, H 6.11, N 4.85; found: C 70.79, H 6.02, N 4.81. (Cbz)2–Pt(bpy)–C60. Yield: 39%. 1H NMR (400 MHz, C2D4Cl2, 75 °C): δ/ppm 9.93 (d, J = 5.7 Hz, 1H), 9.67 (d, J = 5.7 Hz, 1H), 8.69 (s, 1H), 8.19 (d, J = 6.3 Hz, 1H), 8.12 (d, J = 7.7 Hz, 4H), 8.09 (s, 1H), 7.66 (t, J = 7.9 Hz, 4H), 7.45 (d, J = 8.3 Hz, 4H), 7.40 (m, 9H), 7.26 (m, 4H), 5.43 (s, 1H), 5.20 (d, J = 9.6 Hz, 1H), 4.26 (d, J = 9.6 Hz, 1H), 3.19 (m, 1H), 2.78 (m, 1H), 2.53 (s, 3H), 2.51 (m, 1H), 2.01 (m, 1H), 1.92 (m, 1H), 1.72 (m, 1H), 1.58 (m, 1H), 1.46 (m, 3H), 1.29 (bs, 12H), 0.88 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): [M − H]+ 1826.6. Elemental analysis calcd (%) for C125H61N5Pt·3/2CH3OH·1/2CHCl3: C 78.80, H 3.51, N 3.62; found: C 78.65, H 3.71, N 3.99. (tBuCbz)2–Pt(bpy)–C60. (Yield: 33%). 1H NMR (400 MHz, C2D2Cl4, 90 °C): δ/ppm 10.01 (d, J = 4.1 Hz, 1H), 9.79 (d, J = 4.1 Hz, 1H), 8.58 (s, 1H), 8.20 (d, J = 5.5 Hz, 1H), 8.15 (s, 4H), 7.95 (s, 1H), 7.75 (t, J = 7.5 Hz, 4H), 7.51 (d, J = 7.5 Hz, 9H), 7.42 (d, J = 6.8, 4H), 5.34 (s, 1H), 5.26 (d, J = 9.6 Hz, 1H), 4.36 (d, J = 9.6 Hz, 1H), 3.24 (m, 1H), 2.83 (m, 1H), 2.60 (s, 3H), 2.55 (m, 1H), 2.09 (m, 1H), 2.01 (m, 1H), 1.77 (m, 1H), 1.67 (m, 1H), 1.50 (s, 39H), 1.34 (bs, 12H), 0.94 (t, J = 6.8 Hz, 3H). FAB-MS (m/z): [M − 2H]+ 2049.5. Elemental analysis calcd (%) for C141H93N5Pt·CH3OH·1/2CHCl3: C 79.83, H 4.58, N 3.27; found: C 79.89, H 4.95, N 3.55.
Acknowledgements V.W.-W.Y. acknowledges support from The University of Hong Kong and the URC Strategic Research Theme on New Materials. This work was supported by a grant from the Theme-Based Research Scheme (TBRS) of the Research Grants Council of the Hong Kong Special Administrative Region, China (Project no. T23-713/11). We are grateful to the Information Technology Services of The University of Hong Kong for providing computational resources.
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